1. Field of the Invention
This invention relates to organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction. One of the ingredients in the compositions is a polyorganosiloxane containing silicon-bonded fluorinated hydrocarbon radicals and at least two silicon-bonded alkenyl radicals per molecule. More particularly, this invention relates to curable organosiloxane compositions of this type that exhibit excellent adhesion to a variety of substrates that the compositions are in contact with during curing.
2. Background Information
A shortcoming of many organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction is the relatively poor adhesion of these compositions to inorganic substrates such as metals and organic substrates such as synthetic organic polymers. One method for improving the adhesion of these compositions is by applying a primer to the surface of the substrate that will be coated with the composition. Alkoxysilanes containing a organofunctional group such as alkenyl, epoxy, amino, acryloxy or methacryloxy that is bonded to silicon by a carbon are frequently used as the active ingredient in these primer compositions. The type of organofunctional group present on the silane is determined at least in part by the type of substrate.
The additional steps of preparing the primer composition and applying it to a substrate can be eliminated by adding the organofunctional silane ingredient of the primer directly to the curable organosiloxane composition as an adhesion promoter.
A preferred class of adhesion promoters are silanes or organosiloxanes containing one or more silicon-bonded alkoxy groups per molecule and at least one functional group that is reactive with the substrate to which bonding is desired. These functional groups include but are not limited to alkenyl radicals, ethylenically unsaturated groups such as 3-methacryoxypropyl and epoxy groups such as glycidoxypropyl. Typical classes of adhesion promoters for use with organosiloxane compositions that cure by a platinum group metal-catalyzed hydrosilation reaction include but are not limited to those described in U.S. Pat. No. 4,087,585 (issued May 2, 1978); U.S. Pat. No. 4,082,726 (issued Apr. 4, 1978); U.S. Pat. No. 4,311,739 (issued Jan. 19, 1982); U.S. Pat. No. 4,677,161 (issued June 30, 1987); U.S. Pat. No. 4,701,503 (issued Oct. 20, 1987) U.S. Pat. No. 4,732,932 (issued Mar. 22, 1988) and U.S. Pat. No. 4,786,701 (issued Nov. 22, 1988). The curable compositions described in these patents contain a polyorganosiloxane with at least two alkenyl radicals per molecule, an organohydrogensiloxane and a hydrosilation catalyst that is typically a metal from the platinum group of the periodic table or a compound of such a metal.
The hydrocarbon radicals that are bonded to the silicon atoms of the alkenyl-substituted polyorganosiloxanes and organohydrogensiloxanes described in the aforementioned patents relating to adhesion promoters are typically defined as containing from 1 to 10 carbon atoms and being substituted with fluorine or other halogen atom, even though the examples are typically limited to polyorganosiloxanes and organohydrogensiloxanes containing methyl radicals as the silicon-bonded hydrocarbon radicals.
The present inventor discovered that little if any additional adhesion is obtained using some known adhesion promoters when the silicon atoms adjacent to the silicon atoms containing a vinyl or other alkenyl radical are bonded to a fluorinated hydrocarbon radical such as 3,3,3-trifluoropropyl.
Curable organosiloxane compositions containing at least one polydiorganosiloxane having terminal vinyl-substituted siloxane units and diorganosiloxane units with silicon-bonded fluorinated hydrocarbon radicals such as 3,3,3-trifluoropropylsiloxane units are described in U.S. Pat. No. 4,988,758, which issued to T. Fukuda et al. on Jan. 29, 1991. The characterizing feature of these polydiorganosiloxanes is the presence of an average of at least two dimethylsiloxane units between the vinyl-substituted silicon atom and the nearest silicon atom containing a fluorinated hydrocarbon radical. These compositions are converted to cured materials by reaction of the polyorganosiloxane with an organic peroxide or with an organohydrogensiloxane in the presence of a platinum-containing hydrosilation catalyst.
The advantage alleged in U.S. Pat. No. 4,988,758 for compositions containing the type of polydiorganosiloxane described in this patent is a faster cure rate relative to compositions containing polyorganosiloxanes wherein the silicon atoms containing the alkenyl radicals are adjacent to silicon atoms containing fluorinated hydrocarbon radicals. This patent does not discuss the subject of adhesion or recommend the use of any ingredients to improve the adhesion to substrates that are in contact with the organosiloxane composition during curing.